Several types of memory devices, such as Flash memories, use arrays of analog memory cells for storing data. Each analog memory cell stores a quantity of an analog value, also referred to as a storage value, such as an electrical charge or voltage. The storage value represents the information stored in the cell. In Flash memories, for example, each analog memory cell holds a certain amount of electrical charge. The range of possible analog values is typically divided into regions, each region corresponding to one or more data bit values. Data is written to an analog memory cell by writing a nominal analog value that corresponds to the desired bit or bits.
Some memory devices, which are commonly referred to as Single-Level Cell (SLC) devices, store a single bit of information in each memory cell, i.e., each memory cell can be programmed to assume two possible memory states. Higher-density devices, often referred to as Multi-Level Cell (MLC) devices, store two or more bits per memory cell, i.e., can be programmed to assume more than two possible memory states.
Flash memory devices are described, for example, by Bez et al., in “Introduction to Flash Memory,” Proceedings of the IEEE, volume 91, number 4, April, 2003, pages 489-502, which is incorporated herein by reference. Multi-level Flash cells and devices are described, for example, by Eitan et al., in “Multilevel Flash Cells and their Trade-Offs,” Proceedings of the 1996 IEEE International Electron Devices Meeting (IEDM), New York, N.Y., pages 169-172, which is incorporated herein by reference. The paper compares several kinds of multilevel Flash cells, such as common ground, DINOR, AND, NOR and NAND cells.
Eitan et al., describe another type of analog memory cell called Nitride Read Only Memory (NROM) in “Can NROM, a 2-bit, Trapping Storage NVM Cell, Give a Real Challenge to Floating Gate Cells?” Proceedings of the 1999 International Conference on Solid State Devices and Materials (SSDM), Tokyo, Japan, Sep. 21-24, 1999, pages 522-524, which is incorporated herein by reference. NROM cells are also described by Maayan et al., in “A 512 Mb NROM Flash Data Storage Memory with 8 MB/s Data Rate”, Proceedings of the 2002 IEEE International Solid-State Circuits Conference (ISSCC 2002), San Francisco, Calif., Feb. 3-7, 2002, pages 100-101, which is incorporated herein by reference. Other exemplary types of analog memory cells are Floating Gate (FG) cells, Ferroelectric RAM (FRAM) cells, magnetic RAM (MRAM) cells, Charge Trap Flash (CTF) and phase change RAM (PRAM, also referred to as Phase Change Memory—PCM) cells. FRAM, MRAM and PRAM cells are described, for example, by Kim and Koh in “Future Memory Technology including Emerging New Memories,” Proceedings of the 24th International Conference on Microelectronics (MIEL), Nis, Serbia and Montenegro, May 16-19, 2004, volume 1, pages 377-384, which is incorporated herein by reference.
Some known methods and systems define specific orders or sequences of programming arrays of analog memory cells, in order to reduce the level of interference in the cells. For example, PCT International Publication WO 2007/146010, whose disclosure is incorporated herein by reference, describes a method for programming a non-volatile memory array comprising a plurality of memory cells. Each cell is adapted to store a lower and an upper page of data. The lower page of predetermined memory cells is programmed with first predetermined data, and the upper page is programmed with second predetermined data. The upper page of the predetermined memory cells is re-programmed with the second predetermined data.
U.S. Patent Application Publication 2003/0002348, whose disclosure is incorporated herein by reference, describes a non-volatile memory system having an array of memory cells with at least one storage element each is operated with a plurality of storage level ranges per storage element. A flash electrically erasable and programmable read only memory (EEPROM) is an example, wherein the storage elements are electrically floating gates. The memory is operated to minimize the effect of charge coupled between adjacent floating gates by programming some cells a second time after adjacent cells have been programmed. The second programming step also compacts a distribution of charge levels within at least some of the programming states.
U.S. Pat. No. 6,781,877, whose disclosure is incorporated herein by reference, describes techniques of reducing erroneous readings of the apparent charge levels stored in a number of rows of memory cells on account of capacitive coupling between the cells. All pages of a first row are programmed with a first pass, followed by programming all pages of a second adjacent row with a first pass, after which the first row is programmed with a second pass, and then all pages of a third row are programmed with a first pass, followed by returning to program the second row with a second pass, and so on, in a back-and-forth manner across the rows of an array. This technique is said to minimize the effect on the apparent charge stored on rows of memory cells that can occur by later writing data into adjacent rows of memory cells.
U.S. Patent Application Publication 2008/0049506, whose disclosure is incorporated herein by reference, describes a method for programming a set of storage elements. The set is programmed beginning with a word line WLn adjacent to a select gate line for the set. After programming the first word line, the next word line WLn+1 adjacent to the first word line is skipped and the next word line WLn+2 adjacent to WLn+1 is programmed. WLn+1 is then programmed. Programming continues according to the sequence {WLn+4, WLn+3, WLn+6, WLn+5, . . . } until all but the last word line for the set have been programmed. The last word line is then programmed.
U.S. Patent Application Publication 2007/0253249, whose disclosure is incorporated herein by reference, describes a method of programming a nonvolatile memory device comprising a plurality of n-valued nonvolatile memory cells arranged in a matrix, wherein n is a natural number greater than or equal to two. The method includes programming i-valued data to three or more memory cells contiguously arranged along a first direction of the matrix before programming (i+1)-valued data to any of the three or more memory cells, wherein i is less than n, and wherein the three or more memory cells are programmed during three or more respectively distinct program periods, and after programming the i-valued data to the three or more memory cells, programming (i+1)-valued data to a particular memory cell among the three or more memory cells.
Other programming orders for multilevel Flash devices are proposed by Takeuchi et al., in “A Multipage Cell Architecture for High-Speed Programming Multilevel NAND Flash Memories,” IEEE Journal of Solid-State Circuits, volume 33, issue 8, August, 1998, pages 1228-1238, which is incorporated herein by reference.